The present invention relates to a particle handling apparatus for handling particles in a fluid by acoustic radiation pressure in a wide variety of purposes, including the confinement or fusion in a fluid of particles, such as plastic particles and granular polymeric substance particles or bioparticles, such as microorganisms, tissue cell sections, ova or sperm, the trapping of the above-mentioned particles, the transportation in a liquid of the above-mentioned particles, the concentration or removal of the above-mentioned particles, and furthermore, applications to the diffraction and scattering of light by controlling the arrangement of particles of different indices of refraction, all by utilizing a force exerted on the particles when they are subjected to the acoustic radiation pressure in a fluid.
For trapping a particle, in addition to the conventional means of physically making a direct catch with microtweezers, a microneedle or the like, attempts have been made to create means of indirectly trapping a particle by use of a force potential wall produced by electromagnetic waves, light, or acoustic waves.
The means of trapping with tweezers or a microneedle, though this is an easy, simple and secure means, physically contacts a particle and therefore, the size of a particle that can be trapped depends a great deal upon the machining accuracy and the machining limit of the tweezers, microneedle, or the like. In addition, since this method traps a particle by direct contact, there is a possibility of destroying or deforming the particle. Furthermore, when moving a particle that is trapped, the tweezers, microneedle or the like is moved mechanically, and therefore, the particle cannot be moved with higher accuracy than the driving accuracy of a stepping motor or the like.
The trapping means utilizing a force potential wall generated by electromagnetic waves is effective in trapping a particle in which an electric charge can be induced. The electric fields can be superposed one over another. Therefore, by using an adequate superposition method, a gradient force field can be produced with relative ease in any position, so that a particle can be trapped by an electric attraction or repulsion with accuracy exceeding the limit of machining. The notable features of the indirect trapping method using a force potential wall are that there is no risk of destruction or deformation of a particle by contact, that the magnitude of the trapping force can be varied freely by controlling the generating sources of the force potential wall, and that the particle can be set free by putting out the force potential wall. A problem with this method is that the force potential wall is generated by superposition of the electric fields, so that this method is not effective for trapping electrically neutral particles.
The trapping means by generating the force potential wall by a light beam has been used in an attempt to trap neutral particles and dielectric particles. By focusing a laser beam, a force potential wall is generated which has a maximum gradient force at the focal point maximum, and when a gradient force exceeding the scattering force of the incident laser light acts on a particle, this particle can be trapped. A problem with this method is that the trapping means is based on equilibrium between the scattering force and the gradient force, and therefore, for particles whose radius is sufficiently larger than the wavelength, the scattering force changes greatly depending on the shape of the particle. Accordingly, the applicability of this method is limited to the particles, in which the optical scattering does not change much with the direction of trapping. In this method, since the laser light is focused strongly in order to obtain a sufficient gradient force, an irreversible damage by light is done to the dielectric particles and biological particles. Furthermore, when optical measurement is performed, or on substances with sensitivity to light, this method cannot be used. In addition, it is necessary to build a large system, including a light source for generating a powerful laser, or devices, such as a device for guiding the laser beam from the light source to the focal point.
Trapping means utilizing a force potential wall by ultrasonic waves has been used in an attempt to trap polystyrene beads and eggs of a frog. There are various methods for generating a force potential wall. For example, in a paper in Journal of Acoustical Society of America, vol. 89, No. 5 (1991), pp. 2140-2143, it has been reported that in a solution, by two collimated focusing ultrasonic beam generators, ultrasonic waves of 3.5 MHz is produced continuously to form a force potential wall, and at the focal point, the author succeeded in trapping a 0.27-mm polystyrene bead. The author also succeeded in moving a particle in any desired direction by mechanically moving the ultrasonic wave generators. Additionally, with the particles trapped by ultrasonic waves of 3.5 MHz, he also succeeded in moving those particles about 1 .mu.m, though only in the axial position, for each change of 0.01 MHz by varying the frequency of the electrical voltage applied to the transducers.
ACUSTICA Vol. 5 (1955) introduces in pp. 167-178 theoretical calculations and an experiment as to the behavior of particles in plane stationary waves and plane progressive waves of ultrasonic waves.
This paper shows that the factors causing particles to move a node or a loop of the acoustic radiation pressure consist of the sound velocity and the density of particles. The paper theoretically explains that if the particle's density-compressibility factor F (.sigma., .lambda.)&gt;0, the particles gather at the position of a loop and if F (.sigma., .lambda.)&lt;0, the particles gather at the position of a node, and demonstrates that the experimental results agree with the theoretical predictions in an experiment conducted using bubbles in water.
In this paper, the author predicted that the colloid of toluene, benzene or the like in water gathers at the position of a node of the acoustic pressure, while the colloid of nitrobenzene, mercury or the like in water gathers at the position of a loop of the acoustic pressure, he made this prediction from the densities and the sound velocity of those substances.
On the other hand, with regard to the method of generating an ultrasonic focal field having a controlled expanse, as shown in IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 39, No. 1, (1992), pp. 32-38, an attempt has been successfully achieved to generate an acoustic field having a field pattern of the m-th order Bessel function type field pattern around the focal point in front of the disk-shaped shell by arranging ultrasonic wave generators (transducers) in circumferential direction on the shell with a spherical curvature and providing the respective elements by ultrasonic waves with a phase distribution of 2 m.pi. around the whole circumference.